Severe performance degradation of writev
David Rothenberger
daveroth@acm.org
Wed Jul 9 17:19:00 GMT 2014
Corinna Vinschen wrote:
> On Jul 9 09:44, David Rothenberger wrote:
>> Corinna Vinschen wrote:
>>> On Jul 7 12:10, Corinna Vinschen wrote:
>>>> On Jul 7 07:28, jojelino wrote:
>>>>> 2008-07-27 Corinna Vinschen <corinna@vinschen.de>
>>>>>
>>>>> * fhandler_socket.cc (fhandler_socket::send_internal):
>>>>> Send never more then 64K bytes at once. For blocking
>>>>> sockets, loop until entire data has been sent or an error
>>>>> occurs. (fhandler_socket::sendto): Drop code which sends on
>>>>> 64K bytes. (fhandler_socket::sendmsg): Ditto.
>>>>>
>>>>> This commit added workaround for KB823764. but it has
>>>>> brought another performance issue when writev sends <64k of
>>>>> data.
>>>>
>>>> That's why the code contains that FIXME comment. If you have
>>>> a good idea for simple code to split a message into the
>>>> least number of pieces to minimize the number of WsaSendTo
>>>> calls...
>>>
>>> I took a stab at the code and I think the new version improves
>>> writing multiple small buffers a lot. In my testing it still
>>> works in other scenarios, too, but I would be very grateful if
>>> somebody could have a critical look into my code changes as
>>> posted in
>>> https://cygwin.com/ml/cygwin-cvs/2014-q3/msg00003.html
>>>
>>> I uploaded a new developer snapshot to
>>> http://cygwin.com/snapshots/ Please give it a thorough try.
>>
>> FWIW, this snapshot fixed a recent performance degradation for me
>> when doing ssh/rsync transfers within my local network at work.
>> These transfers had run at about 25 MB/s but recently degraded to
>> about 500 kB/s. The snapshot restored the original performance.
>
> Cool. Is the result still intact? It's kind of simple to have
> lots of performance if the code just doesn't send everything... :}
Details, details. :)
Yeah, I just tried a transfer and verified the checksum of the
transferred file. It worked fine.
--
David Rothenberger ---- daveroth@acm.org
Hempstone's Question:
If you have to travel on the Titanic, why not go first class?
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